1. Field of the Invention
The present invention relates to a network system for data communication, and more particularly to update processing of routing information concerning the determination of communication routes in a network system.
2. Description of the Related Art
FIG. 8 shows a logical configuration of a conventional network system of this type. In the drawing, end systems (E11, E12, E21, . . . ) are connected to a network A, a network B, and a network C, respectively. An intermediate system R11 is connected to the networks A and B, and data can be transferred between these networks via a shortest route. An intermediate system R12 is connected between the networks A and C, and data can be transferred between these networks via a shortest route.
Here, in a case where data is to be transmitted from the end system E11 to the end system E21, for instance, the end system E11 requests the intermediate system R11, which transfers data between the networks A and B via a shortest route, to effect routing, and the data is transmitted to the end system E21 via the intermediate system R11.
To transmit data via a shortest route in the above-described manner, each end system stores within itself shortest routes from the network, to which it is connected, to other networks. Each end system requests the routing of data to the intermediate system which selects a shortest route, as necessary, in correspondence with a destination end system and provides the shortest route selected. Such shortest routes between networks are determined by the intermediate systems, and are broadcast from the intermediate systems to the end systems as routing information. Each end system stores the routing information in a routing table. In addition, since the logical configuration of the network system changes with time, the routing information is updated one item after another, and is periodically broadcast. Accordingly, the end systems receive the routing information periodically, and stores and manages most up-to-date information, respectively.
If an end system is unable to receive the broadcast of the routing information for a certain period of time due to some reason or other, the information stored in the routing table which the end system possesses is not updated and, hence, becomes outdated. As a result, its value as information is lost. In addition, that end system will continue to possess this outdated information.
For that reason, a routing information update processing system has been proposed in which routing information which the end system possesses can be updated as the end system itself issues a request for updating the routing information (refer to Unexamined Japanese Patent Publication (Kokai) Hei-2-143759/(1990)).
However, according to the processing system of Unexamined Japanese Patent Publication Hei-2-143759/(1990), since all of the routing information is held, the amount of routing information becomes very large when the size of the network system is expanded or complicated. For this reason, there have been drawbacks in that it becomes difficult for the end system to secure a storage area for storing the routing information, and that the management cost becomes high.
In contrast, a routing information management processing system has been proposed in which necessary routes are extracted by the end system from the routing information sent from the intermediate system, and only the extracted routes are stored and managed (refer to Unexamined Japanese Patent Publication Hei-2-143746/(1990)). According to this processing system, since only the necessary routes extracted from the routing information are stored by the end system, it is possible to reduce the information storage area. Nevertheless, since the validity of the information which is held cannot be judged, the problem that the updated wasteful information is held remains unresolved.
In addition, in the above-described processing systems, the end system waits for periodical reception of routing information unless accurate routes are stored, so that there has been a drawback in that time is required until data is transmitted.